This invention relates to magnetic resonance (MR) methods. More specifically, this invention relates to an off-center field-of-view (FOV) technique for MR imaging.
The magnetic resonance phenomenon has been utilized in the past in high resolution magnetic resonance spectroscopy instruments by structural chemists to analyze the structure of chemical compositions. More recently, MR has been developed as a medical diagnostic modality having applications in imaging the anatomy, as well as in performing in-vivo, noninvasive spectroscopic analyses. As is now well known, the MR phenomenon can be excited within a sample object, such as a human patient, positioned in a homogeneous polarizing magnetic field, by irradiating the object with radio-frequency (RF) energy at the Larmor frequency. In medical diagnostic applications, this is typically accomplished by positioning the patient to be examined in the field of an RF coil having usually a cylindrical geometry, and energizing the RF coil within an RF power amplifier. Upon cessation of the RF excitation, the same or a different coil, such as a surface coil, is used to detect the MR signals emanating from the patient volume lying within the field of the RF coil. In the course of a complete MR scan, a plurality of MR signals are typically observed. The signals are used to derive MR imaging or spectroscopic information about the object studied.
In typical studies, the MR signal is usually observed in the presence of pulsed linear magnetic field gradients used to encode spatial information into the signal. Pulsed magnetic field gradients are also employed with selective RF pulses to excite nuclear spins in predetermined regions of the object undergoing examination. In the course of an MR examination, it is frequently desirable to apply pulsed magnetic field gradients, designated G.sub.x, G.sub.y and G.sub.z, in each of the x, y, and z directions, respectively, of a conventional Cartesian coordinate system. In practice, the direction in which the magnetic field gradient pulses may be applied is not limited in any manner, and any direction could be selected as required.
In conducting an MR imaging study, the patient region of interest must be positioned in the polarizing field and centered about a system isocenter where the magnetic field produced by the magnet is most homogeneous. This is accomplished by using a patient support device capable of bidirectional longitudinal travel into and out of the magnet bore, but which generally is not capable of travel in the transverse or vertical directions within the bore. In some MR imaging situations, it is necessary to apply off-center FOV techniques to select the image object regions which are situated away from the system isocenter. The off-center FOV techniques are used because of limitations on displacing the object studied in the transverse or vertical directions within the bore of the magnet. Therefore, these techniques ease the positioning constraints by allowing the imaged region to be centered at the anatomy of clinical interest, rather than requiring the anatomy to be repositioned at the isocenter. This capability is especially valuable for surface coil imaging, where most of the imaged regions are located near the outer surface of the patient and are displaced relative to the slice-selection axis.
In two-dimensional Fourier transform MR imaging, the three orthogonal gradients identify the three functional imaging axes: The slice-select axis (typically the z axis), the read-out axis (typically the x axis), and the phase-encoding axis (the y axis). Since in general the system isocenter is defined by the point where the three orthogonal magnetic field gradients intersect and have zero amplitude, the off-center FOV techniques can be conveniently classified using the x, y and z axes as references. An offset in the direction of an axis perpendicular to the plane of the image slice (z-axis offset) can be achieved by applying a gradient along that axis along with a frequency selective RF excitation pulse. Only the slice with Larmor frequencies within the range of frequencies contained in the RF excitation pulse will be excited and thus selected for imaging. Other slices offset from the slice centered at the isocenter can be easily excited by appropriately modulating the RF excitation pulse. The offset in the direction of the readout axis (typically the x axis) is relatively easily achieved because the readout axis is frequency encoded by the application of a linear readout gradient. The field of view in the direction of the readout axis corresponds to an encoding frequency band centered at the receiver frequency. Thus, the off-center field of view may be obtained by offsetting the receiver frequency from the isocenter frequency along this axis; that is, the center of the RF excitation pulse frequency. An offset in the phase-encoding direction (typically the y axis) may be accomplished by using selective 180.degree. RF excitation pulses along with a gradient applied in the phase-encoding direction. Such a technique is disclosed in commonly assigned pending application Ser No. 555,097, which is incorporated herein by reference. In a manner similar to that described above with reference to slice selection, the amplitude of the gradient is matched with the frequency of the 180.degree. RF pulse to obtain the desired field of view. The center of the RF pulse thus corresponds to the center of the field of view. By offsetting the center frequency of the RF pulse, the off-center field of view along the phase-encoding axis is obtained. Since this technique requires the 180.degree. RF excitation pulses to be uniquely selective in the direction of the phase-encoding axis, it precludes the use of selective 180.degree. RF pulses for other applications, such as, for example, two-dimensional multi-slice imaging.
It is therefore a principal object of the present invention to provide a method for obtaining off-center field-of-view images, while preserving the availability of the selective 180.degree. RF pulses for use in multi-slice MR imaging or other applications requiring such selective RF pulses.